This invention relates to a control system for a pulse width control type inverter.
One example of an application of the pulse width control type inverter involves providing voltage regulation for an available source of electric current. Since such a source is connected to a load computer, electric communication apparatus or the like, it is desirable to have apparatus for providing a fast transient response when an external disturbance such as the variations in DC voltage and load are experienced.
As diagrammatically shown in FIG. 1, a conventional pulse width control type inverter comprises four semiconductor switching elements 1 through 4, such as power transistors and thyristors, which are ON-OFF controlled in a predetermined sequence for converting the power of a DC source E into alternating current which is applied to an AC load 5. In order to maintain the voltage across load 5 at a constant value irrespective of variations in the DC voltage E, the conduction periods of the switching elements 1-4 are controlled in accordance with the magnitude of the DC voltage, or the conduction periods of switching elements 2 and 4 alone are controlled while the conduction periods of the switching elements 1 and 3 are maintained fixed.
FIG. 2 shows one example of a control system which controls the conduction periods of the switching elements 1-4 and comprises a reference pulse generator 11 generating pulses having a frequency which is an integer multiple (in this example, twice) of the output frequency of the inverter for determining the frequency thereof, a sawtooth wave generator for generating a signal synchronized with of the reference pulses, a constant voltage control circuit 13 for producing an analogue signal having a level corresponding to the DC voltage for the purpose of obtaining a desired output voltage, an analogue comparator 14 which compares the output of the sawtooth wave generator with the output of the constant voltage control circuit 13, a ring counter 15 which in response to the output of the comparator 14 forms a pulse signal adapted to ON-OFF control the switching elements 1-4 shown in FIG. 1 in a predetermined sequence, and a pulse amplifier 16 which amplifies the output of the ring counter 15 to a level that can directly ON-OFF control the switching elements 1 to 4.
FIG. 3A shows waveforms useful to explain the operation of the control system shown in FIG. 2. A, B and C show the waveforms of voltages at corresponding portions of the system shown in FIG. 2. The dotted line level of curve B corresponds to the output of the constant voltage control circuit 13. While this level is higher than that of the output of the sawtooth wave generator 12 the output C of the analogue computer 14 becomes "1" and when the output level of the sawtooth wave generator 12 is higher than the output of the constant voltage control circuit 13, the output C becomes "0". During an interval in which the output C of the analogue comparator 14 is "1" a control signal is applied to the inverter through ring counter 15 and pulse amplifier 16 for rendering ON the switching elements 1-4 in a predetermined sequence thus producing an alternating current output as shown by curve D. In this manner, the conduction period .theta. of the switching elements 1 to 4 vary depending upon the output level of the constant voltage control circuit 13. Accordingly, the output voltage shown by curve D is controlled such that the product EX.theta. of the DC voltage E and the conduction period .theta. is always constant. Therefore, it is possible to always obtain a constant output voltage regardless of the variation in the DC voltage E.
The control system described above, however, can not provide an efficient control by following the output of the constant voltage control circuit when the DC voltage varies rapidly as occurs when switching is made between a commercial source and a battery source. FIG. 3B shows enlarged views of portions of curves B and D shown in FIG. 3A which are useful to explain the operation of the control system when the DC voltage varies rapidly. In FIG. 3B, lines a, b and c of curve B show the output of the constant voltage control circuit 13. Since lines a, b and c interesect the output of the sawtooth wave generator 12 at the same level, the conduction periods .theta. are the same. Even when the constant voltage control circuit 13 has no internal delay and is provided with a DC voltage detecting means for improving its control ability, when the DC voltage varies as shown by curve D-b, FIG. 3B, the output of the constant voltage control circuit 13 varies as shown by curve B-b, FIG. 3B. When the DC voltage varies as shown by curve D-c, the output of the constant voltage control circuit varies as shown by curve B-c. In each case, however, the conduction period .theta. is the same as for the circumstance where the DC voltage is constant as shown by curve D-a. Accordingly, when the DC voltage is constant, product EX.theta. is also constant, but when the DC voltage varies as shown by curve D-b, a deficiency shown by hatched portion 1 and having an area of .DELTA.E.sub.1 .times..theta./2 appears as an error, whereas when the DC voltage varies as shown by curve D-c. a surplus shown by hatched portion 2 and having an area of .DELTA.E.sub.2 .times..theta./2 also appears as an error. Thus, with the control system described above when the rate of variation of the output of the constant voltage control circuit is large, it is difficult to obtain a high response speed. While the foregoing description is made with respect to the variation of the DC voltage, the same difficulty also arises when the output voltage varies due to a rapid variation of the load.
Where the control system is constructed by using analogue techniques, the sawtooth wave generator 12 usually comprises an integrator in the form of an operational amplifier, and it is necessary to use complicated circuit for the purpose of compensating for the characteristics of electronic circuit components such as diodes and transistors, and the off-set voltage and temperature drift of the operational amplifier. Moreover, it is not easy to adjust the inclination and linearity of the sawtooth wave. Also, the analogue comparator 14 is generally constructed with an operational amplifier but this circuit is also difficult to adjust due to the characteristics of the circuit components. Furthermore, the operation of the comparator is affected by noise superposed upon the input signal. Accordingly, it is necessary to use a complicated circuit for overcoming these defects.